The present invention relates to a method of regenerating a solution and, more particularly, to a method of regenerating an acid etching solution which is enhanced with a salt-free material.
Cupric chloride solutions have conventionally been used to etch printed circuits. The chemical formulation for cupric chloride is Cu.sup.(+2) Cl.sub.2. The usual etching procedure involves placing a resist pattern over a sheet of copper laminated to one or both sides of a plastic/glass cloth core. The masked copper laminate is then brought into contact with the etching solution which dissolves the exposed copper and leaves behind the copper which is protected by the resist pattern. When the cupric chloride etches away the copper from the laminated substrate, cuprous chloride is produced in accordance with the following etching reaction: EQU Cu.sup.(0) +Cu.sup.(+2) Cl.sub.2 .fwdarw.2Cu.sup.(+1) Cl
In order to continue the etching process, the cuprous chloride must be regenerated to the cupric chloride form before it is again suitable as an etching agent. To regenerate cuprous chloride, it must be oxidized.
U.S. Pat. No. 3,306,792 issued to Thurmel et al teaches a regeneration method in which etching and regeneration occur in separate devices. Oxidation of a salt-based (ammonium chloride) cupric chloride solution is accomplished by sparging air into a separate vessel containing the solution.
U.S. Pat. No. 3,705,061 issued to King teaches an apparatus for continuously regenerating an alkaline etch solution used to dissolve copper from substrates. The continuous reduction/oxidation (redox) process is accomplished by means of spraying the alkaline etch solution countercurrently with air into a reaction vessel.
These oxidation processes are very inefficient for regeneration of large volumes of dissolved copper. Typically the reactions occur at an unacceptable rate for industry.
Other typical methods for regeneration (oxidation) include the addition of chlorine gas in accordance with the following regeneration reaction: EQU Cu.sup.(+1) Cl +1/2 Cl.sub.2 .fwdarw.Cu.sup.(+2) Cl.sub.2
or by the addition of liquid hydrogen peroxide in accordance with the following regeneration reaction: EQU Cu.sup.(+1) Cl +1/2 H.sub.2 O.sub.2 .fwdarw.Cu.sup.(+2) Cl.sub.2 +H.sub.2 O
Either of the aforementioned last two methods can regenerate large capacity etching systems, some of which being capable of etching at a rate of up to 12,000 grams of copper/hour. The speed of these regeneration processes is due to the great speed with which both chlorine and hydrogen peroxide react with cuprous chloride.
There are, however, serious drawbacks associated with the use of conventional methods of regenerating cuprous chloride when industrial environments are contemplated. One of the drawbacks involves handling difficulty. Chlorine gas is typically delivered in 2,000 lb. containers. Great effort must be exercised to move such volumes of the poisonous gas safely.
Toxicity poses a second drawback. Concentrated hydrogen peroxide (30%) that must be added to bulk systems represents a safety hazard to humans for two main reasons: not only is hydrogen peroxide highly corrosive to skin, but contamination of hydrogen peroxide can lead to its rapid decomposition and violent explosion.
It would be advantageous to provide a method of regenerating metal etching solutions safely.
It would further be advantageous to provide a method of regenerating metal etching solutions in an efficient manner for large scale manufacturing operations.
It would also be advantageous to provide a method of regenerating cupric chloride etching solutions without endangering human welfare.
It would further be advantageous to provide a method of regenerating cupric chloride etching solutions by the use of a packed reaction vessel.
It would also be advantageous to provide a method of regenerating cupric chloride etching solutions by introducing an oxygen containing gas cocurrently with spent acid solution.
Air oxidation of a cupric chloride solution has never been preferred by industry because of its processing inefficiencies. The equation of an air regeneration reaction is: EQU 2Cu.sup.(+1) Cl +2HCl +1/2 O.sub.2 .fwdarw.2Cu.sup.(+2) Cl.sub.2 +H.sub.2 O
The oxidation reaction using air is much slower than that of chlorine because oxygen is less soluble than chlorine in cupric chloride solution. Moreover, the air oxidation reaction mechanism involves several intermediate steps unlike chlorine oxidation which is a direct bi-molecular reaction.